The fabrication of deep submicron Ultra-Large Scale Integrated (ULSI) circuits requires long interconnects having small contacts and small cross-sections. Copper is preferred over aluminum as the conductive element.
Many problems, however, are encountered in fabricating circuit interconnects with copper. Some of the major difficulties include: (a) the rapid diffusion of the copper to the silicon and silicon dioxide of the base; (b) the susceptibility of the copper to oxidize during interconnection fabrication; and (c) the poor adhesive characteristics of copper with dielectric films.
Barrier layers have been used to avoid the aforementioned problems. A layer of titanium nitride (TiN) has been suggested as a possible diffusion barrier due to its inert and conductive nature.
Although reactive sputtering is a common method of generating titanium nitride layers, this method provides very poor step coverage. In multi-level metallization processes utilizing copper, film conformality is critical. This is especially so for high aspect ratio contact, or with the use of holes.
Low Pressure Chemical Vapor Deposition (LPCVD) can provide the titanium nitride films with conformality approaching 100%, as taught by U.S. Pat. No. 4,570,328, issued to Price et al. Unfortunately the titanium nitride film produced by this process has a high resistivity resulting from the residual chlorine within the film.
More recently, it has been proposed to provide a conductive and conformal barrier layer for the copper, by a self-encapsulating process. An anisotropic etching technique using molybdenum has been proposed to encapsulate the copper. D. Gardner et al, IEEE V-MIC, 99 (1991).
Still another proposed process suggests alloying the copper with titanium, and then annealing it in a nitrogen atmosphere at 800.degree. C. The high temperature annealing will chemically form a titanium nitride layer on the copper surface. K. Hosino et al, IEEE V-MIC, 226 (1989). This high temperature technique, however, is not practical for many applications.
The present invention is for a low temperature, self-encapsulating method of producing the copper interconnects in ULSI circuits.
The self-encapsulating process of the invention is characterized by three distinctive steps:
(a) alloying the copper with titanium for deposit as interconnect lines upon the substrate;
(b) after depositing the interconnect lines on the substrate, utilizing a fast heating rate to anneal the lines, so that surface diffusion dominates over internal nucleation, thus providing a surface rich in titanium; and
(c) utilizing an annealing ambient of ammonia below approximately 650.degree. C., to provide chemical formation of the titanium nitride film about the copper interconnect lines.
The critical step in the above-mentioned process is the heating rate of the copper/titanium alloy. The heating ramp which controls the quality of the resulting titanium nitride surface must be approximately between 60.degree. to 80.degree. C./minute, or higher.
The resulting titanium nitride film completely encapsulates the copper interconnect lines and exhibits good adhering properties to the copper. The encapsulating film also has resistance stability after in situ oxidation in air at 200.degree. C.